No Breakup and seemingly low plate voltage

benabloom · Feb 19, 2009

Hi ,

I have a Bandmaster Reverb from '69, that has been modified with a Hoffman style turretboard and AB763 spec'ed, exceot that it has the earlier bias modulating trem (sounds great btw). Now the board was purchase stuffed from the airtightgarage and looks pretty, however I did not independantly varify the values of each component. This has been a long running project amp.

All of the Transformers are original and it is now living in a 2x10 cabinet (like a vibroluxe reverb) with 2 weber 10f150 speakers wired to = 4ohms total.

My issues with the amp as it is is that I cannot get any breakup at all even on 10. stays clean and loud. I tried biasing the tubes hotter and cooler and while the tone changes, there is no tube distortion happening.

Also of note, with a Gz34 recto I am measuring about 420v on the plates, and with a 5u4gb recto I am getting a measily 400v on the plates. Are these voltages indicative of a bad power transformer? something else on the circuitboard seem off with these readings?

I would like the amp to breakup earlier and it does not seem normal to me as it is. I am up for the challenge... now what should I do , look for etc?

-Ben

TweeDLX · Feb 20, 2009

First thing that comes to my mind is checking the cathode resistor & bypass cap values, filter caps, and coupling caps against the original schematic. Plate resistors? A place to start, if nothing else.

Mike

SatelliteAmps · Feb 20, 2009

420v doesn't seem like a bad transformer. Check to make sure your bias cap is in correctly (positive to ground). Then start taking some voltage measurements on the preamp and power tubes and give us a chart.

Blue Strat · Feb 20, 2009

What kind of bias readings are you getting, what are your preamp tube plate voltages, have you swapped in different tubes? Plate voltage sounds a little bit low. What's your line voltage and has the plate voltage changed since before the mod? Lower plate voltage will get you earlier breakup , not later.

If the bias and tubes are all good, I suspect a wiring or component problem which is causing too little preamp gain.

benabloom · Feb 20, 2009

SatelliteAmps said: ↑

420v doesn't seem like a bad transformer. Check to make sure your bias cap is in correctly (positive to ground). Then start taking some voltage measurements on the preamp and power tubes and give us a chart.
Click to expand...

Ok will do on Sunday when I have some time to get the amp opened up again. I'll post back then!

Blue Strat said: ↑

What kind of bias readings are you getting, what are your preamp tube plate voltages, have you swapped in different tubes? Plate voltage sounds a little bit low. What's your line voltage and has the plate voltage changed since before the mod? Lower plate voltage will get you earlier breakup , not later.

If the bias and tubes are all good, I suspect a wiring or component problem which is causing too little preamp gain.
Click to expand...

Bias readings are coming in fine on the power tubes. Biased at about 40ma with 420 on the plates currently. I have not checked preamp voltages yet. Can I use my bias rite to check preamp voltages as well, or do I have to crack her open and use a multimeter off the pins. My line voltage is about 120. I don't know if the plate voltage changed after the mod, because the amp was in hacked up/non working condition when I recieved it.

I will check the voltages on the preamp and power tubes and report back! Thanks for the help guys!

-Ben

Structo · Feb 20, 2009

According to the schematic, that circuit used solid state rectifiers.
You are going to drop more voltage with a tube rectifier than with a full wave solid state.
The PT was probably designed with 115v on the wall so you will get approx 5% more voltage out of it at 120v.

The schematic shows 440 on the plates. But given the tolerances of the various components which were generally 20% that could be as much as 10% either way.

The 5U4 drops more voltage than the GZ34.

Are you using a 7025 or 12ax7 in V1?

What do you have in the PI spot?

For 420v on the plates you can have a bias current range of 35ma (50%) to 50ma (70%) .

benabloom · Feb 20, 2009

Structo said: ↑

According to the schematic, that circuit used solid state rectifiers.
You are going to drop more voltage with a tube rectifier than with a full wave solid state.
The PT was probably designed with 115v on the wall so you will get approx 5% more voltage out of it at 120v.

The schematic shows 440 on the plates. But given the tolerances of the various components which were generally 20% that could be as much as 10% either way.

The 5U4 drops more voltage than the GZ34.

Are you using a 7025 or 12ax7 in V1?

What do you have in the PI spot?

For 420v on the plates you can have a bias current range of 35ma (50%) to 50ma (70%) .
Click to expand...

I thought the Bandmaster Reverb TFL5000 used a 5U4GB recto stock...
I have a mullard 12at7 in the PI and 12ax7 in both v1 and v2.

Swarty · Feb 20, 2009

If you are using regular output single coils you are not going to get much grind.

benabloom · Feb 20, 2009

Swarty said: ↑

If you are using regular output single coils you are not going to get much grind.
Click to expand...

Im using WCR Fillmore Humbuckers high output pickups. I also own a few fender's and clones (71 Deluxe reverb, 70 super reverb, allen old flame )
so I am fairly familier with the type of breakup I should be getting.

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